In the document and product security industry, it is desirable to have a low-cost anti-counterfeiting device that can be used easily by any consumer. Ideally, the device is inexpensive to make but impossible to duplicate, and easy to verify but applicable to large numbers of different documents and products. Although it is probably not possible to meet those ideals, the various embodiments of the present invention do provide a low-cost anti-counterfeiting device that is very difficult to duplicate or reverse engineer, and that has an appearance visibly different from other optically variable anti-counterfeiting devices. In fact, a polarizing filter can allow easy observation of the distinguishing features of an anti-counterfeiting device made according to the various embodiments of the present invention.
The effective refractive index of a grating layer on a surface depends on the grating orientation and grating profile, as well as on the bulk properties of the material composing the layer. Specifically, when the grating period is significantly smaller than the wavelength of light incident on the grating, the effective refractive index is higher if the electric field vector of the light is aligned with the grating lines rather than being perpendicular to the grating lines. If the material composing the grating layer is metallic, the reflectivity of the layer depends on the alignment of the electric field vector relative to the grating lines. In the first case, the grating layer is birefringent, and in the second case the grating layer is a reflective polarizing filter. Both are referred to herein as cases of “form birefringence”.
A common problem in the fabrication of gratings is “ghosting,” which are periodic errors in the position or depth of the grating across a surface. For example, in electron-beam fabricated gratings, ghosting can be caused by uneven friction in drive screws, round-off errors in positioning algorithms, or finite step size in stepping motors. A very high frequency (short period) grating that is free of ghosting will show no diffraction effects. However, if ghosting is present with a period greater than half of the wavelength of visible light, diffraction effects appear, showing rainbow colors in white light. The polarization effects of the high-frequency grating are still apparent, however, in direct reflection (zero-order diffraction).
Form birefringence has been demonstrated and reported by several researchers, and is exploited commercially. For example, Nano-Opto Corporation sells a “Subwave Polarization Beam Splitter/Combiner” and a “Broadband Polarizer For Optical Networking Applications” that employ surface relief gratings having a period smaller than an optical wavelength.